1. Field of the Invention
The present invention relates to a screw compressor unit.
2. Description of the Related Art
In a screw compressor, a reaction force of compressed gas acts on a suction side in an axial direction against a screw rotor. Therefore, the screw compressor is provided with a thrust bearing for receiving a thrust load of a rotor shaft. When a discharge pressure of the screw compressor increases, a thrust force caused by the reaction force of gas also increases, thus resulting in shortening the life of the thrust bearing.
Therefore, Japanese Patent Laid-Open No. 2002-168185 and Japanese Patent Registration No. 4050657 disclose a technique in which a balance piston connected to a rotor shaft is fitted in a cylinder, pressure of a fluid supplied to the cylinder is applied to the balance piston, and the rotor shaft is pressed to a discharge side in an axial direction.
While Japanese Patent Laid-Open No. 2002-168185 discloses an invention in which the balance piston is pressed by pressure of gas discharged by a screw compressor, a difference between a thrust force caused by a reaction force of gas and a thrust force generated by the balance piston is generated by a variation of a suction pressure of the screw compressor.
Japanese Patent Registration No. 4050657 discloses an invention in which the balance piston is pressed by pressure oil supplied from a hydraulic pump. There, a control valve whose opening is adjusted according to suction/discharge pressures of the screw compressor is provided in an oil supply passage to adjust pressure of the pressure oil applied on the balance piston, thereby adjusting the magnitude of the thrust force generated by the balance piston.
However, a change in the thrust force caused by the reaction force of gas does not depend only on the suction/discharge pressures. For example, in a screw compressor whose capacity is adjusted by changing an opening position of a rotor chamber by means of a slide valve, the magnitude of the thrust force caused by the reaction force of gas is changed according to a position of the slide valve even if the suction pressure and the discharge pressure are the same.
In consideration to the above problems, the present invention is to provide a capacity-adjustable screw compressor capable of appropriately cancelling a thrust force generated by a balance piston and a thrust force caused by a reaction force of gas each other.